There are two basic techniques for implementing analog-to-digital converters (ADCs), the open-loop technique and the feedback technique. An open-loop converter generates a digital code directly upon application of an input voltage, and is generally asynchronous in operation. A feedback converter generates a sequence of digital codes from an input signal, reconverts these digital codes to an analog signal.
Sigma-delta ADCs use the feedback technique and have been known in the industry since the early 1960s. The sigma-delta technique is attractive because it achieves high resolution by precise timing instead of precisely matched on-chip components, such as resistors and capacitors used in open-loop converters. Thus, the sigma-delta technique is the technique of choice for many integrated circuit applications.
A basic sigma-delta ADC receives an analog input signal, and subtracts a feedback signal from the analog input signal to provide an error signal. The error signal is processed through a lowpass filter, and then quantized to form a digital output signal. A feedback digital-to-analog converter (DAC) provides the feedback signal after converting the digital output signal to analog form. Aside from the feedback DAC, the basic sigma-delta ADC may be implemented with conventional analog components such as operational amplifiers, comparators, and switched-capacitor filters. The basic sigma-delta ADC usually provides high resolution because integrated circuit clocking speeds allow the analog input signal to be highly oversampled. The basic sigma-delta ADC also has high signal-to-noise ratio (SNR) because the lowpass filter shapes quantization noise out-of-band, which can then be sufficiently attenuated by conventional filtering techniques.
While the basic sigma-delta ADC is easy to implement in conventional integrated circuit processes and generally has high performance, it still is not ideal for some applications. For example, a frequency modulation (FM) radio receiver receives a radio frequency (RF) signal, mixes the RF signal down to an intermediate frequency (IF), and mixes the IF signal to baseband to provide in-phase (I) and quadrature (Q) signals, and processes the I and Q baseband signals. A digital FM receiver may use the ADC to convert the IF analog signal to digital form for further digital signal processing.
However an FM receiver using the basic sigma-delta ADC has problems. The frequency band of the analog input signal may be relatively small (300 kilohertz (kHz) for entertainment FM radio) in relation to the IF center frequency (10.6875 megahertz (MHz), e.g.), which may in turn be relatively large compared to the sampling frequency. Thus the resulting oversampling ratio may well be insufficient to yield a digital signal with sufficient resolution. One technique to overcome these problems is to use two basic sigma-delta ADCs with inputs sampled out-of-phase. However, the ADCs in such a receiver create gain and phase errors and offsets due to component mismatch in the lowpass filter. Thus what is needed is an improved sigma-delta ADC for applications such as FM radio receivers and the like.